Lies, on their own, are not sufficient evidence of a crime though in some situations they may themselves be a crime—making false statements, fraud, false advertising, perjury. However, lies may indicate that the defendant is aware of one's own guilt, and the prosecution may rely on the fact that the defendant has lied as supplementary to other evidence.

In England, if the prosecution seeks to rely on the fact that the defendant lied (for example, to police), it is sometimes necessary for the judge to give a direction to the jury (known as a Lucas direction).[1]

The direction is not used where the prosecution attempts to show that the defendant committed the crime, and, if the jury find the defendant guilty, this would mean the defendant had lied. The direction "comes into play when the prosecution say, or the judge envisages that the jury may say, that the lie is evidence against the accused; in effect using it as an implied admission of guilt .... this is quite distinct from the run of the mill case in which the defence case is contradicted by the evidence of the prosecution witnesses in such a way as to make it necessary for the prosecution to say ... that the defendant's account is untrue and indeed deliberately and knowingly false".[2] It is appropriate for a judge to give a Lucas direction:[2]

Where the judge considers it desirable or necessary to suggest that the jury should look for support or corroboration of one piece of evidence in the case, and amongst that other evidence draws attention to lies told, or allegedly told, by the defendant.

Where the prosecution seek to show that something said, either in or out of the court, in relation to a separate and distinct issue was a lie, and to rely on that lie as evidence of guilt in relation to the charge which is sought to be proved.

Where although the prosecution has not adopted the approach (in (iii) above) the judge reasonably envisages that there is a real danger that the jury may do so.

The judge should tell the jury that the lie is only evidence of guilt if they are satisfied it was made deliberately.

The judge should remind the jury that people might lie not because they are guilty but for other reasons (for example, to bolster a weak case, to protect someone, out of panic or to cover up disgraceful behaviour).

The judge should tell the jury that the lie alone is insufficient evidence and they should look to see if the other evidence corroborates guilt.